The present invention relates to a method of producing an in-mold coated thermoplastic workpiece. More specifically the process comprises the steps of forming a substrate by injection molding a thermoplastic polymeric material and thereafter injecting, as soon as the substrate falls below its melt point, a coating material into the closed mold containing the substrate which is maintained at a constant clamp pressure to coat at least a portion of one of the surfaces of the substrate with the coating material. The substrate and in-mold coating are introduced into the mold using known injection molding techniques.
The in-mold coating, which is sometimes referred to as IMC, of molded plastic parts, particularly automobile and truck body panels, is a widely used commercially acceptable method of production. Up to the present these commercially accepted items have almost all been made by compression molding using a fiberglass reinforced plastic (FRP) substrate. The most widely used processes are those described in U.S. Pat. No. 4,076,788. The in-mold coating materials generally used therewith are of the type described in U.S. Pat. Nos. 5,658,672; 5,614,581; and 5,777,053.
The parts that have been manufactured using the above-described processes and materials have generally been relatively large and relatively flat. This is due in part to the inherent constraints of applying a coating to a compression molded part and has limited what might be a very useful method to relatively few parts.
Until relatively recently there have been no commercially acceptable in-mold coating injection molding techniques. More recently, however, an application describing an injection molding technique and the in-mold coating used in the process was developed by some of the inventors of this invention and is more fully described in pending U.S. patent application Ser. No. 09/614,953.
Another in-mold coating process which contemplates injection molding is described in U.S. Pat. No. 6,180,043B1. That in-mold coating method uses multi-stagewise variable clamping pressures. The scenario in changing pressures in this process is time consuming and, accordingly, decreases the throughput of the molding machine on which it is being practiced. In other words, machine throughput, i.e. number of articles produced per unit of time, is not maximized.
The present invention is a process for producing a thermoplastic workpiece having a coating bonded thereto, comprising the steps of injecting, using a filling pressure, into a closed mold which is maintained under a constant clamping pressure, a thermoplastic material, such as a polyolefin, heated to a temperature above its melting point, until said mold is substantially full, completely filling said mold with said material using a packing pressure to form a workpiece; maintaining said thermoplastic material, as it cools, under a mold pressure; injecting, immediately after the workpiece cools to its melt temperature or as it is sometimes referred to melting point, a coating composition into the closed mold to contact at least a portion of a surface of the workpiece. The mold is opened and the workpiece is removed after the coating composition has at least partially cured.
A process for the production of substrates of a thermoplastic having in-molded coatings thereon has been developed. In-mold coating of a substrate or workpiece, whereby the coating composition has good flow and coverage during molding, good adhesion, uniform color, good surface quality, and, if necessary, good paintability, may be successfully achieved by the practice of the process of the present invention.
It is an object of the present invention to provide an injection molding process by which substrates may be coated with in-mold compositions, to form finished workpieces which are suitable for use as is in end use applications or which require minimal surface post-treatment.
Another object is to maximize the output of expensive injection molding equipment.
It is a further object of the present invention to eliminate the time and cost of pretreating a workpiece to accept a paint or other coatings thereon.
A further object of the present invention is to provide a workpiece having an appearance in-mold coating thereon, which has paint-like properties, such as high gloss, hardness, good adhesion and good weatherability.
A further object of the present invention is to provide a workpiece having an in-mold coating thereon, which has good flow and coverage during molding, good adhesion, uniform color, durability, weather resistance, good surface qualities, and good paintability.
Injection molding is a well known and probably the most widely used method of producing plastic parts. In a typical process pelletized, granular or powdered plastic material is fed from a hopper into a heating cylinder. There it is softened by being forced through the heated cylinder, usually by a screw. The softened plastic is then injected into a closed mold, most often by using the screw as a ram. Pressure is maintained on the mold and on the plastic until the plastic reaches a state where it can be removed from the mold without distortion.
The mold into which the plastic is injected is in two parts; one stationary, and the other movable. The mold cavity generally has a first surface on the first mold half, upon which a show or finished surface of the molded article will be formed, and a corresponding or opposite second surface on the second mold half. The mold is opened and closed either mechanically or hydraulically usually using a predetermined timing cycle. The stationary half normally houses the cavity section of the mold and is mounted on the stationary platen in contact with the injection section of the cylinder of the injection machine. The movable mold half usually holds the core and the ejector mechanism. The injection of the plastic material occurs under pressure when the mold is in a closed position. The clamping pressure, that is the pressure used to keep the mold closed during the injection of the plastic must be greater than the pressure used to inject the plastic.
Injection molding machines are often rated according to the maximum number of ounces of uniformly heated plastic that can be injected into the mold with one complete stroke of the injection ram. Shot sizes typically range from about ten to 260 ounces but may be smaller or larger. Another method of measuring machine capability is the clamp force, usually in tons, available to hold the mold closed during the high pressure injection. Usual injection molding pressures range from 10,000 to 30,000 psi.
Most injection molding machines are horizontal but some are of the vertical type. Another machine variation is a so called two stage injection unit.
Another essential component of the machine is the clamp assembly which opens and closes the mold and ejects the finished part and further prevents the mold from opening during the pressure build up resulting from the injection of the material to be molded into the mold cavity. The clamping devices used today may be either mechanical, hydraulic or hydromechanical. The type most often used is a toggle clamp. In this set up, prior to injection, mechanical links in the clamp are collapsed or untoggled and the mold is opened. Pressure is then applied forcing the links to extend and then close the mold and at its fullest extension the linkage is in a position such that pure mechanical pressure holds the mold closed. Hydroelectric clamps and hydromechanical clamps may also be used.
The invention may be practiced using any of the various types of injection molding machines provided that provision is made to inject the in-mold coating.
The practice of this invention requires the application of a second polymeric material generally referred to as an in-mold coating (IMC) onto at least a portion of the substrate which was molded as described above. The additional equipment needed to apply it is a second injector, the IMC injection nozzle of which is preferably located within the tool parting line and on either mold half, and preferably on the mold half opposite the ejector systems and thermoplastic injection gates or sprues. The mold cavity also contains separate orifices to allow the first and second composition injectors to inject their output into the mold. The injector may be located in the movable mold half or the stationary mold half. The IMC is injected directly through a nozzle into the mold cavity and onto a surface of the substrate. In some instances due to the complexity of the substrate more than one nozzle may be required to inject either or both the substrate polymer and IMC. During the entire molding operation it is essential that the mold be maintained in a tightly closed, i.e. locked position so that there can be no leakage of either the substrate or IMC.
Close control of the processing variables is essential for successful molding. Machine controls accurately govern such functions as temperatures, times, speed, hydraulic and melt pressures and component positions. This is usually accomplished using microprocessors and microcomputers which allow integration of the various machine functions, which will be discussed in some detail below, and to a single system control and monitoring set up which handles all of the operations of the clamp, the injection unit, the injector mechanism as well as some ancillary equipment.
As discussed in more detail below injection molding can be carried out with virtually all thermoplastic resins.
The process of the present invention utilizes in-mold coatings, many of which are available commercially. Such coatings include GenGlaze(copyright) and Stylecoat(copyright), appearance in-mold coatings available from Omnova Solutions Inc. as well as others. These and other coatings are well known to the art. The main advantage of acrylic coatings is the high degree of resistance to thermal and photoxidation and to hydrolysis, giving coatings that have superior color retention, resistance to embrittlement and exterior durability. Low-molecular weight acrylic resins having an average functionality of two to three and contain few molecules that are nonfunctional or only monofunctional, are useful in the present invention.
Epoxy resins are also useful in the present invention. A principal use of epoxy resins is as a component in two-package primer coatings. One part contains the epoxy resin and the other part contains a polyfunctional amine. Amine-terminated polyamides, sometimes called amido-amines, are widely used. A preferred epoxy resin is an epoxy-based oligomer having at least two acrylate groups and at least one copolymerizable ethylenically unsaturated monomer, and at least one copolymerizable monoethylenically unsaturated compounds having a xe2x80x94COxe2x80x94, group and a xe2x80x94NH2xe2x80x94, NH, and or xe2x80x94OHxe2x80x94 group.
The present invention also contemplates the use of other resin coatings, such as alkyds, polyesters, urethane systems, amino resins, phenolic resins, and silicone resins. See e.g., Kirk Othmer, Encyclopedia of Chemical Technology, Vol. 6 (4th ed. 1993) at pp. 676-690.
In-mold coatings comprising five components, namely
1) a saturated aliphatic polyester intermediate urethane
2) an aliphatic polyether
3) an aliphatic or cycloaliphatic portion (meth)acrylate
4) hydroxy alkyl (meth)acrylates
5) vinyl substituted aromatics
have been found to have particular utility in the practice of this invention. In-mold coating compositions useful in the practice of the invention are prepared as follows. The polyester urethane acrylate is mixed with the vinyl substituted aromatic monomers such as styrene, the saturated aliphatic or cycloaliphatic (meth) acrylates such as isobornyl acrylate, and the hydroxyalkyl methacrylate, such as hydroxypropyl methacrylate. After these compounds are mixed, fillers and additives, such as cure inhibitors, light stabilizers, lubricants, etc., are added and mixed. The free radical generating initiator is added last. The polyacrylate ester of a polyol can be present in the polyester urethane acrylate from the supplier. This in-mold coating composition is clear after curing.
Any of the coatings contemplated for use in the present invention can be colored by utilizing a pigment, a colorant, etc., in a desired or effective amount to yield a desired color, tint, hue, or opacity. Pigments, pigment dispersions, colorants, etc. are well known to the art and include, for example, graphite, titanium dioxide, carbon black, phthalocyanine blue, phthalocyanine red, chromium and ferric oxides, aluminum or other metal flake, and the like.
When an in-mold coating having a specific color is desired, one or more pigments, colorants, etc., can be utilized in suitable amounts. As known to the art, often times various pigments or colorants are added with a carrier, for example, a polyester, so that they can be easily blended. Any suitable mixing vessel can be utilized, and the various components and additives mixed until the compounds are blended. Even if pigments are not contained in the blend, the mixture at this point is not clear.
All of the above-described in-mold coating compositions that may be utilized in the present invention may contain other additives and fillers, etc., in amounts known to the art. For example, various cure inhibitors such as benzoquinone, hydroquinone, methoxyhydroquinone, p-t-butylcatechol, and the like, can also be utilized. Other additives may include an accelerator, such as cobalt octoate. Other classes of accelerators include zinc, or other metal carboxylates. Various light stabilizers can also be utilized such as, for example, the various hindered amines (HALS), substituted benzophenones, and substituted benztriazoles, and the like. Lubricants and mold release agents are generally utilized with specific examples including various metal stearates, such as zinc stearate or calcium stearate or phosphonic acid esters. Reinforcing fillers, such as talc, can be utilized. Other additives include hardeners, thixotropes, such as silica, and adhesion agents, such as polyvinyl acetate.
Some of the in-mold coatings contemplated by the present invention are chain extended through the utilization of a free radical initiator, such as a peroxide. Examples of suitable free radical initiators include tertiary butyl perbenzoate, tertiary butyl peroctoate in diallyl phthalate, diacetyl peroxide in dimethyl phthalate, dibenzoyl peroxide, di (p-chlorobenzoyl) peroxide in dibutyl phthalate, di (2,4-dichlorobenzoyl) peroxide in dibutyl phthalate dilauroyl peroxide, methyl ethyl ketone peroxide, cyclohexanone peroxide in dibutyl phthalate, 3,5-dihydroxy-3,4-dimethyl-1,2-dioxacy-clopentante, t-butylperoxy (2-ethyl hexanoate), caprylyl peroxide, 2,5-dimethyl-2,5-di (benzoyl peroxy) hexane, 1-hydroxy cyclohexyl hydroperoxide-1, t-butyl peroxy (2-ethyl butyrate), 2,5-dimethyl-2,5-bis (t-butyl peroxy) hexane, cumylhydroperoxide, diacetyl peroxide, t-butyl hydroperoxide, ditertiary butyl peroxide, 3,5-dihydroxy-3,5-dimethyl-1,2-oxacyclopentane, and 1,1-bis (t-butyl-peroxy)-3,3,5-trimethyl cyclohexane and the like, and mixtures thereof. It is sometimes desirable to use mixtures of initiators to take advantage of their different decomposition rates and times at different temperatures and so forth. A preferred initiator to use is tertiary butyl perbenzoate.
Azo-initiators useful for the non-aqueous application of this invention include: 2,2xe2x80x2-azobis (2,4-Dimethylpentanenitrile); 2,2xe2x80x2-azobis (2-Methylpropanenitrile); 2,2xe2x80x2-azobis (2-Methylbutanenitrile); 1,1xe2x80x2-azobis (Cyclohexanecarbonitrile); 2,2xe2x80x2-azobis (4-Methoxy-2,4-dimethyl-valeronitrile); Dimethyl-2,2xe2x80x2-azobisisobutyrate; 2-(Carbamoylazo)-isobutyronitrile; 2,2xe2x80x2-azobis (2,4,4-Trimethylpentane); 2-Phenylazo-2,4-dimethyl-4-methoxy-valeronitrile); and 2,2xe2x80x2azobis (2-methylpropane).
The initiators should be used in an amount sufficient to overcome any effect of any inhibitors used and to cause curing of the ethylenically unsaturated compounds. In general, the peroxide initiator is used in an amount of up to about 5% or from about 0.25 to about 5%, desirably from about 0.5 to about 2%, and preferably from about 0.5 to about 1%, by weight, based on the total weight of all of the ethylenically unsaturated components employed in the in-mold coating compositions.
The process of the present invention contemplates a reaction of the in-mold coating compositions, in the presence of an initiator. In the present process, activation temperatures of the initiators used are less than the melt temperature of the substrate. These initiators do not xe2x80x9ckick offxe2x80x9d the free radical initiator until after the IMC is injected into the closed mold containing a formed substrate. At that time the substrate has cooled to a temperature below its melt point.
There is a relationship between the melt temperature of the thermoplastic used as the substrate and the half life of the initiator used in the in-mold coating. The half life at a particular temperature of the initiator must be such that it institutes the reaction of the in-mold coating at a temperature below the melt temperature of the substrate thermoplastic while enabling the reaction to go to substantial completeness before the coated workpiece is removed from the mold.
The resins useful as substrates in the practice of the invention are manifold but must be thermoplastic. The only requirement is that the substrate resin be amenable to being injection molded in commercially available equipment. Resins useful in the practice of the invention include PET or polyethylene terephthalate, polystyrene, PBT or polybutylene terephthalate and PBT alloys, polypropylene, polyurethane, ABS or acrylonitrile-butadiene-styrene copolymer, PVC or polyvinyl chloride, polyesters, polycarbonates, PP/PS or polypropylene polystyrene alloys, polyethylene, nylon, polyacetal, SAN or styrene acrylonitrile, acrylics, cellulosics, polycarbonate alloys and PP or propylene alloys. Other combinations of these materials may be used. The foregoing list is not meant to be exhaustive but only illustrative of the various materials useful in the practice of this invention.
Set out below in Table I are the melt temperatures (as reported in Plastics Digest Edition 20, Vol. 1) of a number of thermoplastics useful in the practice of this invention. If mixtures are used or if the melt temperature of a particular polymer is not available it may be determined using ASTM D3418.